1. Field of the Invention
The present invention relates generally to soft handover in a code division multiple access (CDMA) mobile communication system, and in particular, to soft handover in a multimedia broadcast/multicast service.
2. Description of the Related Art
Currently, due to the development of the communication industry, a service provided by a code division multiple access (hereinafter referred to as “CDMA”) mobile communication system is being developed to include multicasting multimedia communication that transmits not only voice service data but also high-capacity data such as packet data and circuit data. In order to support the multicasting multimedia communication, a broadcast/multicast service has been proposed in which one data source provides a service to a plurality of user equipments (hereinafter referred to as “UE”). The broadcast/multicast service can be divided into a cell broadcast service (hereinafter referred to as “CBS”), i.e., a message-based service, and a multimedia broadcast/multicast service (hereinafter referred to as “MBMS”) that supports multimedia data, such as real-time image and voice, still image, and text.
The CBS is a service for broadcasting a plurality of messages to all UEs located in a particular service area. The particular service area where the CBS is provided can be an entire cell area where the CBS is provided. The MBMS is a service for simultaneously providing voice data and image data, and requires many transmission resources. The MBMS is serviced over a broadcast channel, since a plurality of services can possibly be provided at the same time within one cell.
Generally, in an asynchronous mobile communication system, timing synchronization between Node Bs is not provided fundamentally. That is, as the Node Bs have their own independent timers, reference times of the Node Bs can be different from one another. A unit of the timer is called a Node B frame number (BFN). Each Node B can include a plurality of cells, and each cell has a timer that advances at regular intervals from the BFN. A unit of the timer given to each cell is called a system frame number (SFN). One SFN has a length of 10 ms, and the SFN has a value of 0 to 4095. One SFN is comprised of 38400 chips, and one chip has a length of 10 ms/38400.
Therefore, when a radio network controller (hereinafter referred to as “RNC”) transmits MBMS data to Node Bs, if there is no separate synchronization process among the Node Bs (or cells), the respective Node Bs (or cells) will transmit the MBMS data at different times. This means that when a UE moves to a new cell (or Node B), it cannot receive the existing service.
Obviously, a UE roams from one cell area to another cell area rather than staying in one cell area. At this point, a common voice service is continued through soft handover. However, soft handover for the MBMS service has never been defined. Therefore, if a UE that was receiving an MBMS service from a particular Node B in a specific cell area moves to another cell area, the UE cannot continue to receive MBMS data, and must again perform an initialization operation for MBMS in order to receive the MBMS service from a new cell (or Node B).